Electrostatic Precipitators for Air Cleaning

Title: Electrostatic Precipitators for Air Cleaning

Author: Oleg Tchetchel

Article:
Canadian Air Systems Co. is a designer and manufacturer of
industrial dust collection systems. Dust collecting equipment is
available in numerous designs utilizing a number of principles
and featuring wide variation in effectiveness, initial cost,
operating and maintenance expense, space, arrangements and
material of construction.

Factors influencing dust collection equipment selection include:

* Concentration and particle size of contaminant.

In most dust conveying systems, usual dust range from 0.1 to
100 micron - a rather wide range of particle sizes.

* Degree of collection required.

Evaluation will consider the need for high efficiency high cost
equipment such as electrostatic precipirators; high efficiency
moderate cost equipment such as fabric or wet collector units;
and the lower cost primary units such as the dry centrifugal
group. Degree of dust collection required can depend on plant
location; compariosn of quantities of material released to
atmoshere with different type of dust collectors; nature of
contaminant - its salvage value or its potential as a health
hazard, public nuissance or ability to damage property - and the
requirements of the local or state air pollution regulations.
* Characteristics of air or gas stream.

High temperature gas streams exceeding 180 F will prevent the
use of standard cotton media in fabric collectors; presence of
steam or condensation of water vapor will cause packling and
plugging of air or dust packages in fabric and dry centrifugal
collectors. Chemical composition can attack fabric or metal in
dry collectors and cause extremely corrosive conditions when
mixed with water in wet type collectors.

* Characteristics of contaminant.

Chemical composition can cause attack on dust collector
elements or corrosion in wet type dust collectors. Sticky
materials like metallic buffing dust impregnated with buffing
compounds can adhere to collector elements plugging dust
collector passages. Linty materials such as dust from textile
opener, picker and napper will adhere to certain types of
collector surfaces or elements. Abrasiveness of many materials
in moderate to heavy concentrations such as dust from sand
blasting will cause rapid wear particularly on dry type
centrifugal collectors. Particles size and shape will rule out
certain collector designs. The parashute shape of particles such
as "bees wings" from grain will "float" through centrifugal
collectors due to their velocity of fall being slower than much
smaller spherical particles of the same specific gravity.
Combustible nature of many finely divided materials will
influence selection of explosion proof dust collectors for such
products.

* Methods of disposal.
Methods of removal and disposal of collected materials will
vary with the material, plant process, quantity involved and the
collector design. Dry collectors can be unloaded continuously or
in batches through dump gates, trickle valves and rotary locks
to conveyors or containers. Wet collectors can be arranged for
batch removal or continual ejection of dewatered material by
flight conveyors or draining as a slurry. Material
characteristics can influence other problems, such as packing
and bridging of dry materials in dust hoppers, floating of
slurry forming characteristics in wet collectors, etc.

One of the most efficient type of dust collectors is
Electrostatic Precipitator.

The principle of dust collection relies on the ability to impart
a negative charge to particles in the gas stream causing them to
move and adhere to the grounded or positively charged collector
plates. Most precipitators are made for horizontal air flow with
velocities 100 to 600 fpm. The collecting plates or electrodes
are parallel elements, usually on 9-inch centers, and
constructed in various ways including corrugated or perforated
plates for rod curtains. The electrode rods are centered between
the collector plates. Volatge difference between electrode and
plate is 60,000 to 75,000 volts in most designs. Collector
plates of cylindrical shapes surrounding the electrode tod are
provided where water is used to wash off collected material and
where the gas stream is under high pressure or vacuum.
Removal of the collected material is obtained by rapping or
vibrating the elements either continuously or at predetermined
intervals. Vibration or unloading usually takes place without
stopping air flow through the cleaning cycle.

Pressure drop is low. Collection efficiency is high and nearly
uniform regardless of particle size including sub-micron
particles. Space is relatively large and cost is high where
small gas volumed (below 50,000 CFM) are involved due to the
cost of high voltage electrical equipment. Efficiency is
improved with increased humidity of the air stream as a change
takes palce in the dielectric properties of the dust. Heavy
concentrations, on the other hand, cause a reduction in
collection as the space chatge on numerous particles blankets
the corona effect from the electrode.

Electrostatic precipitators have been extensively used in high
temperature gas cleaning from equipment such as blast furnaces,
open heart furnaces and central station pulverized fuel boilers.
The chemical industry has many applications including sulfuric
acids plants, carbon black, cement kilns and soda sh from paper
mill black liquor furnaces. As voltage setting is close to the
spark-over potential, application is limited to materials that
are not explosive or combustible in nature unless the carrier
gas stream is an inert gas.

For additional information please refer to
http://www.nis-co.com/dustcollection/Index.html.

Oleg Tchetchel, Ph.D Process Engineer Canadian Air Systems
http://www.nis-co.com
http://www.nis-co.com/fumeextractor/Index.html

About the author:
Oleg Tchetchel, Ph.D Process Engineer Canadian Air Systems
http://www.nis-co.com
http://www.nis-co.com/fumeextractor/Index.html